Add an nfs backend based on libnfs

Add an nfs backend based on libnfs to make userspace mounting and usage
of nfs shares easier. The backend is written in a single-threaded,
asynchronous style. Performance measurements show around 60-70 MiB/s
throughput on 1GbE.
To make use of it, simply mount the share with gvfs-mount or Nautilus
with the following syntax:
nfs://host/export/path
Authentication is simple, based on UNIX uid.
Since this is a userspace nfs client, it comes with the caveat that the
mount needs to be exported with "insecure" on Linux (or some equivalent
for other NFS servers) so that it allows connections from port numbers
higher than 1023. Alternatively, a special capability can be given to
the binary:
sudo setcap 'cap_net_bind_service=+ep' /path/to/executable
https://bugzilla.gnome.org/show_bug.cgi?id=738967